272 Mr Stevenson’s Experiments on Lights in Rapid Motion. 
of the impression itself; for the continuity of effect must, of 
course, be caused by fresh impulses succeeding each other be- 
fore the preceding ones have entirely faded. If it were other- 
wise, and the time required to make the impression were 
equal to the duration of the sensation, it would obyiously be 
impossible to obtain a series of impulses so close or conti- 
nuous in their effects as to run into and overlap each other, 
and thus throw out the intervals of darkness, because the 
same velocity which would tend to shorten the dark inter- 
yals, would also curtail the bright flashes, and thus prevent 
their acting on the eye long enough to cause an impression. 
Accordingly, we find that the duration of an impression is 
in reality much greater than the time required for produc- 
ing the effect on the retina. It is stated by Professor Wheat- 
stone, in the London Transactions for 1834, that only about 
one millionth part of a second is required for making a dis- 
tinct impression on the eye; and it appears, from a state- 
ment made by Lamé, at p. 425 of his Cours de Physique, that 
M. Plateau found that an impression on the retina preserved 
its intensity unabated during one hundredih of a second, so 
that, however small these times may be in themselves, the one 
is yet 10,000 greater than the other. 
It has been ascertained by direct experiment,* that the eye 
can receive a fresh impression before the preceding one has 
faded ; and indeed, if this were impossible, absolute continui- 
ty of impression from any succession of impulses, however ra- 
pid, would seem to be unattainable ; and the approach to per- 
fect continuity would be inversely as the time required to 
make an impression. 
From this property which bright bodies passing rapidly be- 
fore the eye possess of communicating a continuous impression 
to the sense of sight, Captain Hall conceived the idea, not 
merely of obtaining all the effects of a fixed light, by causing 
a system of lenses to revolve with such a velocity as to pro- 
duce a continuous impressicn, but, at the same time, of obtain- 
* Lamé, Cours de Physique, p. 424. L’impression peut subsister encore 
lorsquela suivante a lieu. 
